Production Riser

ABSTRACT

The present invention is directed to a system including a self supporting riser (SSR) which is connected to a well to produce fossil hydrocarbon reservoirs deep below the seafloor. The SSR is constructed of a plurality of joints comprising regular joints and specialty joints that define the SSR and are selected to optimize the SSR for a well in a specific location. A unique aspect of the SSR of the present invention is that while capable of connecting to the wellhead, or tree on the seafloor, it can also be secured to an anchor during operations. The invention is further directed to a small vessel moored to the SSR by a line such as a hawser, the riser providing an anchor to the vessel, and the SSR carrying fluids from the well to the vessel and from the vessel to the well. The vessel has provisions for processing the fluids from the wellhead.

FIELD OF INVENTION

The present invention is directed to a riser for the production ofhydrocarbons from fossil hydrocarbon reservoirs deep below the seafloor.Further, the present invention is directed to the interfacing of theriser to a vessel subject to high vessel motions of pitch and roll. Thesmall vessel employs a unique stabilization system for the separationprocessing equipment on deck.

BACKGROUND OF THE INVENTION

It has been the practice for the recovery of hydrocarbons from fossilhydrocarbon reservoirs deep below the Gulf of Mexico and other offshoreareas to build platforms of various designs upon which the separationequipment for separating the products from the wells; namely, the liquidhydrocarbons (oil), from water and gas, are supported. These platformstructures cost millions of dollars and can not be cost justified unlessthey service more than one well and the indications have been determinedthere is sufficient oil/gas from the wells to put these structures inplace. The production risers of the present invention may be employed ona newly drilled well even before the extent of the field or hydrocarbonreservoirs are fully developed. Using a low cost production structure,namely a SSR, for the first drilled well permits evaluation of thereservoir without the drilling of additional wells. The SSR must becapable of handling the unexpected as well as the expected.

SUMMARY OF THE INVENTION

The present invention is directed to a system including a selfsupporting riser (SSR) which is connected to a well to provide fluidcommunication to fossil hydrocarbon reservoirs deep below the seafloor.The SSR is constructed of a plurality of joints comprising regularjoints and specialty joints that define the SSR and are selected tooptimize the SSR for a well in a specific location. A unique aspect ofthe SSR of the present invention is that while capable of connecting tothe wellhead, or tree on the seafloor, it can also be secured to ananchor during operations. The invention is further directed to a smallvessel subject to high vessel motions moored to the SSR by a line suchas a hawser, the riser providing an anchor to the vessel, and the SSRcarrying fluids from the well to the vessel and from the vessel to thewell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view a self supporting riser (SSR) connected to awell for testing and producing hydrocarbons from a fossil hydrocarbonreservoir deep below the seafloor;

FIG. 2 is a schematic cross-sectional view of one embodiment of a selfsupporting riser (SSR) illustrating multiple tubulars side by side;

FIG. 3 is a schematic cross-sectional view of another embodiment of aself supporting riser (SSR) illustrating multiple tubulars in aconcentric configuration;

FIG. 4 is a schematic view a self supporting riser (SSR) connected to awell for producing hydrocarbons from a fossil hydrocarbon reservoir deepbelow the seafloor; and a vessel subject to high vessel motions mooredto the SSR with the riser providing an anchor to the vessel;

FIG. 5 is a schematic top view of a novel production vessel havingprocessing equipment on a stabilized frame on the vessel;

FIG. 6 is a schematic side view of a novel production vessel;

FIG. 7 is a top isometric view of a vessel configuration;

FIG. 8 is an isometric view of a stabilized frame on the vessel; and

FIG. 8A is a schematic diagram illustrating a hydraulic system forstabilizing the frame on the vessel.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The present invention is directed to a riser system including a selfsupporting riser (SSR) which is in fluid communication with a well totest and produce fossil hydrocarbon reservoirs deep below the seafloor.Still further the present invention is directed to a self supportingriser (SSR) which is in fluid communication with a well and preferablyincludes a small vessel with processing equipment on a stable frame onthe deck moored to the (SSR) carrying fluids from the subsea well to theprocessing equipment on the vessel.

To substantially lower cost over the prior art, the present inventionuses a small vessel to facilitate operation of processing equipment onboard rather than a multi-million dollar platform or a large vesselhaving large day-rates. It is preferred that the vessel is moored to theSSR so that the small vessel does not require using dynamic positioningto maintain vessel position, further lowing cost.

The present invention uses a SSR to provide fluid communication from awell or seafloor production equipment to the small vessel moored to theSSR rather than a riser fixed to a platform or a large vessel.

The methods and techniques for the SSR design and assembly and placementon an element of a subsea infrastructure are fully set forth in U.S.patent application Ser. No. 12/714,919.

Referring to FIG. 1, a Self Supporting Riser (SSR) 10 is illustrated onan element of a subsea infrastructure, such as a wellhead 20. Whenplaced on the wellhead 20 the SSR 10 provides fluid communication with awell beneath the seafloor. Riser 10 has one or more buoyancy modules 15and 19. The uppermost buoyancy module 19 is referred to herein as thenear-surface buoyancy module and any module below module 19 is referredto herein as a mid-water buoyancy module. At the lower end of the SSR 10is a connector 25 suited to the target wellhead or tree or anotherelement of seafloor infrastructure. Preferably, a Seafloor ShutoffDevice 11 (when used) may be directly above the connector 25. Aspecialty joint 18 with functions of a Blow Out Preventer (BOP) ispreferably below the near-surface buoyancy module 19. Functions of aBlow Out Preventer (BOP) 18 may be to shear tubing or pipe if necessaryand provide a seal to fluids in the riser 10. When there is no tubing orpipe passing through the BOP device this SSD11 can consist of simplyactivating one or more valves.

A specific design of the SSR 10 is only illustrated in FIG. 1; however,the present invention as explained in Ser. No. 12/714,919, also includesprovisions to assemble an SSR for a particular purpose, water depth,current conditions, or location from an inventory of standardized jointsand to recover the joints and assemble some or all of them into adifferent SSR configuration for a different application. Having placedthe SSR 10 on a wellhead or tree 20, the pre-production of the well isaccomplished by opening the well to flow and permitting fluidcommunication to the SSR. One aspect of the present invention is thatthe system of the present invention may allow fluids from the well to amoored vessel over a period of time to determine the material parametersof the reservoir from which the fluids are flowing. While pressure andtemperatures are indicated in the drilling procedures, thesustainability of pressure in the reservoir or the amounts of gas andwater over time flowing from a specific reservoir are preferablymeasured by allowing the flow of fluids from the reservoir over time.

Referring to FIGS. 2 and 3, these Figures illustrate that a riser 10 maybe of multiple tubular construction; FIG. 2 illustrating one or moreside by side tubulars 10.sup.1 and 10.sup.2 with the remaining areaeither empty or filled with insulation and FIG. 3 illustrating one ormore concentric tubulars 10.sup.1 and 10.sup.2 separated by spacers10.sup.3 surrounded by insulation. During the drilling procedures,indication of high pressures in the reservoirs or mixtures of gas andwater that are prone to form hydrates may indicate that a specialconfiguration of the riser is desired. For example, side by side tubularin the riser may be used when the well has more than one tubular in thewell casing such as one tubular extending to a reservoir at one depthand another tubular extending to a reservoir at a lower depth, possiblyfor the purpose of gas reinjection. For example a concentric tubularriser may be used when double containment is required or when heatedwater may be used to heat the fluid flowing in the inner tubular wherehydrates may form.

Riser 10 may be attached to an element of a seafloor infrastructure,such as a wellhead or tree 20 (FIG. 1), throughout production or may beattached to a seafloor anchor 22 such as a pile; or a gravity orembedment anchor (FIG. 4). The lower end of the SSR 10 has provision fora flexible jumper line 26 to connect to the tree 20 for the flow ofhydrocarbons from the well and up through the riser 10. There are alsoprovisions for the control umbilical 12 to extend to the tree 20.

Attachment of the SSR 10 to the seafloor anchor is preferably by aflexible connector 25, such as two half links of chain 25′ which permitinclination in any direction but prevent axial rotation of riser 10.Possible alternatives include a section of flexible pipe which bendswithout buckling or a flexible joint such as is commonly used as ahanger for steel catenary risers. A mechanical connection such as twohalf links of chain allows the SSR to freely incline from vertical atany compass bearing, thus avoiding bending moment in the SSR near theseafloor. Configuring the mechanical connection between the SSR and theanchor to prevent the SSR from rotating about its axis preventsexcessive loads on the flexible pipe 26 shown connected to the well orproduction equipment. A mechanical connection can be simple, or can bemore sophisticated and may include provisions for functions such asconnection and release by ROV or other remote means.

A swivel can be placed at any location in the SSR 10 to allow the vesselto weather van freely without causing excessive torsion in the riser.The production riser 10 preferably has a swivel 24 mounted above thenear-surface buoyancy module 19. Placing a swivel high in the SSR butbelow the buoyancy would require a swivel that functions under hightension. Placing the swivel near the seafloor locates it where SSRtension is low, but subjects the swivel to high ambient pressure andplaces it in a relatively inaccessible location. The swivel 24 (orswivels) is preferably located in the SSR above the load path to thebuoyancy as illustrated to avoid both high tension and the complicationsassociated with placing a swivel near the seafloor. A single swivel 24for flow of fluid between riser 10 and vessel 30, controls (umbilical12), and connecting the mooring line 36 can be placed as shown, orseparate swivels, with or without provisions to avoid mooring linetension on the fluid swivel can be located in the position as shown. Thetorque required to operate the swivel(s) must be less than the torquerating of the SSR and must be less than the torque required to break outany threaded connections in the SSR. Wind direction current frequentlyshift gradually from east to south to west to north, or vice versa. Windshifts such as this can drive a moored vessel multiple times around themooring point, either clockwise or counter clockwise. By use of one ormore swivels, the potentially damaging axial torsion to the SSR andassociated flexible pipe can be avoided.

Mooring of a small vessel 30 can be as shown in FIG. 4, where a mooringline(s) 36 extends from the top of the SSR 10 to a mooring buoy 37 thatfloats on the sea surface, and mooring line(s) 38 extend to the vessel30 to secure the vessel 30 to the upper part of the SSR 10. The mooringline 36 attached to the buoy 37 may be installed as part of assemblingthe production riser 10. A flexible pipe 27 is shown connected from theupper part of the SSR, preferably through the swivel 24, to the vessel30 to provide continuation of the flow path for produced fluids from thewell or production equipment below. Buoyancy 28 may be used to supportand tend the flexible pipe 27.

A small vessel 30, which is subject to relatively high pitch and rollmotions due to its size, is used in the production system of the presentinvention to avoid overloading the SSR 10 as a larger vessel might. Whennot subject to vessel mooring loads the SSR 10 stands upright, subjectonly to self weight and drag due to ocean currents (as illustrated inFIG. 1). When a vessel 30 is moored to the SSR 10 (illustrated in FIG.4), wind and surface current pull the vessel and the top of the SSR awayfrom the otherwise upright position of the SSR until the resultingoffset angle and upward force of the buoyancy create a restoring forceto balance the forces on the vessel to secure and moor the vessel. Whenthe force of the vessel is relaxed or released the SSR restores itselfto the nearly vertical attitude where it can survive untended and beready for subsequent use.

Wind, current, or other forces on the moored vessel 30 pull on thehawser line 38 and move the surface buoy 37 until both line 36 and line38 are taut. The vessel then continues to move and pull on the hawserline 38 until an adequate restoring force is created by pulling theriser off vertical and pulling the surface buoy deeper into the water.The vessel becomes essentially stationary when the restoring force isequal to the force acting to move the vessel. An increase or decrease inforces acting to displace the vessel will cause this geometry to adjustuntil vessel position is again stable.

The horizontal force from a moored vessel 30 pulls the SSR 10 offvertical and consequently causes the top of the SSR to move down to agreater depth below the surface. Because line 36 is not horizontal,tension in this line includes a vertical component which pulls thesurface buoy deeper into the water and increases the tension in theriser. The horizontal restoring force from the buoyancy module 19 isproportional to the total upward force at the top of the riser times thesine of the angle of inclination off vertical of the SSR and thisinclination increases as the SSR is pulled further off vertical.Therefore the restoring force increases as the top of the SSR is pulledfurther from its vertical position. The surface buoy 37 is sufficientlylarge to prevent it from being pulled completely underwater and thelength of line 36 is chosen to achieve the desired relationship betweenriser tension and riser inclination. Line 36 is always long enough toallow the surface buoy to float with freeboard. Beyond this, making line36 longer results in greater maximum inclination of the riser andreduced maximum tension in the riser.

The total horizontal force at the top of the riser must be reacted by ahorizontal force component at the seafloor. The use of a flexibleconnection 25, the preferred embodiment of which is two half links ofchain, allows the horizontal component of riser tension to betransmitted to the anchor 22 without a bending moment in the riser.

The depth of the top of the SSR increases as the SSR is pulled offvertical. If the buoyancy module 19 is a sealed gas can, the pressuredifferential across its hull will increase, and must not be allowed toexceed the rating of the hull. If the buoyancy module 19 is a vented gascan, the gas in it will compress so buoyancy will decrease, and buoyancymust not be allowed to decrease below the required value. In either casethese difficulties can be avoided by using umbilical 12 from the vesselto trim the gas fill of the buoyancy module 19. The umbilical 12 ispreferably dressed with the flexible pipe 27, but can be a separate linefrom the vessel.

When hydrocarbon fluids are brought onboard vessel 30 they must beprocessed prior to transportation. The equipment 29 for this processingtypically must be held reasonably steady to prevent sloshing of thefluids in tanks and to allow the liquid oil to separate from water. Asmall vessel 30, such as can reasonably be moored to an SSR 10 asdescribed above, exhibits relatively large pitch and roll motions forany given sea state. It is not practical to operate a production systemonly when the sea is relatively calm. Therefore practical use of a smallproduction vessel moored as above requires a stabilized support on whichto mount the fluid processing equipment 29. Heave (vertical) motionshave little effect on the process equipment. Surge and sway motions aretypically quite small, but pitch and roll motions require stabilization.With minor adaptation, the pitch/roll stabilization system described inU.S. Ser. No. 12/714,919 can be used here to support a frame upon whichto install process equipment that is sensitive to pitch and roll. In theprocessing system of the present invention; however, the embodiment thathas the stable frame above the cylinders is preferred.

Referring now to FIGS. 5, 6 and 7, illustrate the stable frame 40 on thedeck 31 of vessel 30 that supports the processing equipment 29. Theprocessing equipment 29 is schematically shown without all theconnecting lines as a number of combinations are possible. For example,processing equipment may comprise separator tanks 42 and 43 thatseparate the gas, the liquid hydrocarbons, and water. The gas is removedfrom the top of the tanks 42 and 43 and is compressed and transferred toa gas tank 44. Water settles to the bottom of tanks 42 and 43 and isremoved, purified, and discharged. The liquid hydrocarbons are removedfrom above the water of tanks 42 and 43 and transferred to an oil tank48 which may be below the deck. The combination of the arrangement ofthe equipment 29 includes using one, two, or more tanks 42 and 43 as perexisting industry practice with flow from line 27 into one or moreseparator tanks 42 and 43. The separated oil may be held in tank 48 onvessel 30 or may be transferred to a second vessel or barge (not shown)to be taken to shore. The water may be purified in device 46, or in acentrifuge 49; or treated in a series of centrifuges 49; or be treatedon another vessel before being returned to the sea. This is not intendedto recite all the possible equipment combinations as the specifichydrocarbon and water mixture brought up riser 10 will determine themost suitable combination of equipment.

As shown in FIGS. 8 and 8A frame 40 can be mounted on and supported by 2(two) or more pairs of vertically mounted hydraulic cylinders. It ispreferred that each cylinder be attached to the vessel deck 31 with thecylinder rods upward, and the rods attached by compliant joints 51 tothe frame 40 to accommodate the changes of alignment as the vesselpitches and rolls. Each pair is connected by relatively large diameterhydraulic line connected to the bottom of each pair and a smallerdiameter hydraulic line is connected to the top of each pair ofcylinders. In FIG. 8, for simplicity of explanation, one pair ofcylinders is shown fore (52F) and aft (52A) and the other pair is shownport (53P) to starboard (53S). When the vessel pitches upward in front(see FIG. 8A) inclination of the frame can be avoided by transferringfluid from the fore cylinder 52F to the aft cylinder 52A to make thefore cylinder shorter and the aft cylinder longer. A force is necessaryto accelerate the frame, so any acceleration of the frame results inhigher force on the cylinder where the vessel is rising (52F), andconsequently the pressure in that cylinder goes up causing the fluid toflow to the cylinder located where the vessel is dropping (52A). In africtionless system inertia would thus keep the frame level, so long asthe center of gravity is centered between the cylinders. Active controlis required to overcome friction and supply the associated energy and tocompensate for offset center of gravity.

As shown in FIGS. 8 and 8A, a reversible pump is mounted between eachpair of cylinders, pump 55 between cylinders 52F and 52A and pump 57between cylinders 53P and 53S, the pumps are preferably between thenon-load bearing chambers of the cylinders. This facilitates pumping ata lower pressure and, with the cylinders mounted with the rod ends up,pumping smaller quantities of fluid thus reducing energy consumption andimproving reliability.

A feedback signal from an inclinometer is subtracted from a referencesignal and the resulting error signal is used to control the directionand speed of the pump, inclinometer 58 controlling pump 57 andinclinometer 59 controlling pump 55. The pump thus speeds up as theframe tips along the axis between the pair of cylinders and the pumpslows down and stops as this axis on the frame becomes level. It isapparent that the inclinometers (58′ and 59′) could alternately be fixedwith respect to the deck 31 of the vessel and used to drive the frame 40in the direction opposite the deck's direction of motion. It is alsoapparent that accelerometers with appropriate signal conditioning couldbe used as an alternate or complement to inclinometers, and further thata combination of sensors on the deck and on the frame could be used.

The frame could be mounted on three cylinders (or any odd number ofcylinders) if used with some method of apportioning flow between them tokeep the frame level. Pairs of cylinders are preferred for simplicity ofthe control system. Two pair of cylinders are adequate for the desiredperformance. If there are 3 or more pairs the system will continue tofunction after any failure, so long as two pair remain functional andthe other pair(s) are not locked in place. Therefore, using 3 or morepairs allows operation to continue normally following a failure andallows one pair to be removed from service for maintenance while theremaining pairs continue to keep the frame level.

It is desirable but not necessary to locate the center of gravity of theframe and its load at the midpoint between the cylinders. However,centering the center of gravity is frequently not practical and the loadmay shift during operations. For the system as described here,quiescence with an offset center of gravity requires a higher pressurein the cylinder(s) closer to the center of gravity and a correspondinglylower pressure in the cylinders further from the center of gravity inorder to hold the frame level. This difference (the bias pressure) maybe different for each pair of cylinders. Since each pump is controlledby a signal generated from inclinometers or accelerometers theyautomatically generate the required bias pressure. When transferringfluid from a more heavily loaded cylinder chamber to a less heavilyloaded cylinder chamber it may be necessary for the pump to operate as amotor and deliver energy from the load to the power supply. This can beaccomplished by, for instance, using hydraulic gear pumps which operateas motors if the pressure of the fluid flowing into the gear motor/pumpexceeds the pressure of the fluid flowing out of the device.

1. A riser system comprising: a single self supporting riser (SSR) whichcomprises a plurality of joints comprising regular joints and specialtyjoints that define said SSR and selected to optimize said SSR for aparticular location; at least one specialty joint comprising a buoyancymodule, said one is the uppermost buoyancy module near but below the seasurface and another specialty joint having provisions at the lower endof said SSR to prevent excessive bending moment in the riser; said riserproviding fluid communication between said SSR and a well having awellhead on the seafloor; and a specialty joint in the SSR providingfluid communication between said SSR and a vessel subject to high vesselmotions of heave, pitch and roll.
 2. A riser system according to claim 1wherein said specialty joint in the SSR providing fluid communicationbetween said SSR and a vessel includes a swivel.
 3. A riser systemaccording to claim 1 wherein said SSR is secured to an element of asubsea infrastructure.
 4. A riser system according to claim 1 whereinsaid specialty joint having provisions at the lower end of said SSR toprevent excessive bending moment in the riser is a flexible connector.5. A riser system according to claim 4 wherein said flexible connectoris connected to an anchor on the seafloor.
 6. A riser system accordingto claim 4 wherein said flexible connector is a section of flexiblepipe.
 7. A riser system according to claim 4 wherein said flexibleconnector is links of chain.
 8. A riser system according to claim 2wherein said swivel is above said buoyancy module.
 9. A productionsystem comprising: a self supporting riser (SSR) which comprises aplurality of joints comprising regular joints and specialty joints thatdefine said SSR and selected to optimize said SSR for a particularlocation; at least one specialty joint comprising a buoyancy module,said one is the uppermost buoyancy module near but below the seasurface; said riser in fluid communication with a hydrocarbon producingwell having a wellhead on the seafloor; and a vessel subject to highvessel motions of heave, pitch and roll; said riser providing an anchorto said vessel and providing fluid communication between said well andsaid vessel.
 10. A small sea vessel subject to high vessel motion ofheave, pitch and roll having on deck a hydrocarbon processing systemcomprising: a stabilized frame on which said hydrocarbon processingequipment is held; and a stabilization system consisting of pairedcylinders to maintain said frame essentially level.
 11. A small seavessel according to claim 10 wherein each cylinder is mounted on saiddeck and each cylinder rod extends upward to a compliant connection tosupport said frame.
 12. A small sea vessel according to claim 10 whereinthere are at least two pair of cylinders.
 13. A small sea vesselaccording to claim 10 wherein each pair of cylinders is controlled byaccelerometers or inclinometers.
 14. A small sea vessel according toclaim 10 wherein each pair of cylinders has a hydraulic control systemconsisting of a fluid line between the load bearing chambers and asecond line between the non-load bearing chambers; and a reversible pumpin one of said lines.
 15. A small sea vessel according to claim 14wherein said pump is in said non-load bearing line.
 16. A small seavessel according to claim 14 wherein said pump is controlled byaccelerometers or inclinometers.
 17. A small sea vessel according toclaim 10 including mooring lines to a self supporting riser having aflexible connection at the lower end permitting said SSR to be pulledoff vertical without introducing a high bending moment to said SSR. 18.A small sea vessel according to claim 10 including: lines and a surfacebuoy securing and mooring said vessel to a self supporting riser throughwhich said hydrocarbons are carried from the seafloor to saidhydrocarbon processing equipment.
 19. A self supporting riser (SSR)comprising: a plurality of joints comprising regular joints andspecialty joints that define said SSR and selected to optimize said SSRfor a particular location; at least one specialty joint comprising abuoyancy module, said one is the uppermost buoyancy module near butbelow the sea surface; and a lowermost specialty joint that is aflexible connector.
 20. A self supporting riser (SSR) according to claim19 further including: a specialty joint that is a seafloor shutoffdevice above said connector; and a specialty joint that has blow-outpreventer functions below said buoyancy module.
 21. A self supportingriser (SSR) according to claim 20 wherein said riser is multipletubular.
 22. A self supporting riser (SSR) according to claim 19 whereinsaid flexible connector is connected to a subsea infrastructure.
 23. Aself supporting riser (SSR) according to claim 22 wherein said subseainfrastructure is an anchor.
 24. A self supporting riser (SSR) accordingto claim 19 wherein said flexible connector is a section of flexiblepipe.
 25. A self supporting riser (SSR) according to claim 19 whereinsaid flexible connector is links of chain.
 26. A self supporting riser(SSR) according to claim 19 further comprising: a swivel located at thetop end of the SSR for connection of a line to carry hydrocarbons fromsaid SSR to processing equipment on a stabilized platform on a vessel.27. A self supporting riser (SSR) according to claim 26 wherein saidswivel is above said buoyancy module.
 28. A hydrocarbon productionsystem comprising: a self supporting riser (SSR) which comprises aplurality of joints comprising regular joints and specialty joints thatdefine said SSR and selected to optimize said SSR for a particularlocation; at least one specialty joint comprising a buoyancy module,said one is the uppermost buoyancy module near but below the seasurface; said riser in fluid communication with a hydrocarbon producingwell having a wellhead on the seafloor, and a vessel moored to said SSR;said riser having a specialty joint at the lower end of said SSR toprevent excessive bending moment in the riser to provide an anchor tosaid vessel and providing fluid communication between said well to saidvessel.
 29. A hydrocarbon production system according to claim 28wherein said SSR further includes: a specialty joint that is a seafloorshutoff device near said specialty joint at the lower end to preventexcessive bending moment in the riser; and a specialty joint that hasblow-out preventer functions below said buoyancy module.
 30. Ahydrocarbon production system according to claim 28 wherein said riseris multiple tubular.